1. Field of Invention
The invention concerns casting molds for casting metal, in particular the precision casting of fine parts and processes for production of casting molds which are based on a generative rapid prototyping process, as well as ceramic raw materials particularly suited for this process.
2. Related Art of the Invention
In foundry technology the state of the art in the production of cast parts includes the use of lost wax sand or ceramic molds, which are not reused following casting.
The shaping of casting molds in the case of precision casting, ingot casting or shell casting is generally by a lost wax process or a related processes. Therein first a wax model of the final cast part is produced. The wax model is then coated a number of times with mold sand, which generally involves applying or coating an appropriate ceramic slip. The application of slip is carried out a number of times until the green mold exhibits sufficient shape stability in order to allow melting out of the wax, thereby forming a mold cavity in the casting mold. As a rule, the green casting mold must be fired prior to casting of metal, in order to pyrolyze organic additives (binders in particular) and to solidify the casting mold, turning it into ceramic. The term “turning into ceramic” is to be understood as referring to the firing or sintering of the green mold.
The lost wax process has the disadvantage that the wax model, in the following also referred to as the original model, must be produced by complex and intricate processes. Particularly in the case of producing prototypes various models must be individually constructed of wax. Due to the high thermal coefficient of expansion of wax great tolerance is allowed for dimensional trueness. Also considered disadvantageous therein is the additional process step of melting out the wax which, particularly in the case of relatively large models, translates into a supplemental expenditure in time.
One possibility for entirely dispensing with the manufacture of wax models and at the same time for providing an almost unlimited diversity of individual molds is provided by rapid prototyping processes (RP-process). Processes are known in which ceramic casting molds can be executed directly from computer models using generative RP-processes. Therein the casting molds are built up layer-by-layer with ceramic powder layers, which are built up and solidified, by adhesion or sintering, in selected areas into a three dimensional casting pre-mold. These processes are described in greater detail for example in U.S. Pat. No. 5,204,055. The disadvantage of this process includes that in general a substantially rougher surface is formed than in the case of the lost wax process. Typically, a step-like surface structure is formed, reflecting the thickness of the ceramic powder layers. Further, the complete removal of the residual not bound powder remaining in the inside of the mold leads to problems. This results in a lower surface quality, or to a penetration of the metal casting.
It is proposed in U.S. Pat. No. 6,109,332 to wash the surfaces of the cavities of the casting molds produced by generative RP-processes with solvent, or to flatten the step-like surfaces using a rubbing compound. This process however leads basically also to a removal or, in certain cases, a distortion of the desired precise structure.
A further series of problems is concerned with the dimensional trueness of the casting.
Metallic precision casting is carried out as a rule as a hot casting in which the casting mold is at temperatures of several 100° C. and, as a result of its thermal expansion, assumes a greater volume than in the cold state. After the casting of the metal the ceramic casting mold and the metal cool together from high temperatures to room temperature. As a rule the metal exhibits a substantially higher thermal coefficient of expansion than the ceramic, so that tensions and warping of the foundry casting can occur.
One approach to minimizing tensions and warping is described for example in EP 0 370 751 B1. It is proposed that the metal and ceramic are to be matched as closely as possible with respect to their thermal coefficient of expansion, in that a two-layer casting mold is built up. Therein the inner layer of the casting mold exhibits a higher thermal coefficient of expansion than the outer layer. The inner layer is preferably made of zircon (zirconium silicate) and aluminum oxide flakes. The casting mold is built up by coating wax models (lost wax process). This constructed geometric arrangement of the two layers is not producible by the known generative RP-process. Besides this, the zircon, at 4.5*10−6K−1, exhibits a relatively low thermal coefficient of expansion. The forestalling or retarding of the shrinkage of the cast metal, in particular mold cores, remains as before.
A further substantial influence on dimensional precision is inherent in the firing of the green casting mold. Firing leads to a reduction in volume of the casting mold. Only for the simplest of molds can this shrinkage be pre-calculated and compensated for by appropriate over-dimensioning of the original model.